The invention concerns a multi-layer body having an optically machine-readable identification, a method of manufacturing such a multi-layer body and a method of reading an item of information out of such a multi-layer body.
EP 0 718 795 A1 discloses an information strip with optically encoded information. Optical markings are arranged on the information strip in bit lines, wherein at least two bit lines with identical division into surface portions of equal size are required. Each surface portion is occupied for example by an optical diffraction element. The diffraction elements of the one bit line involve the same grating parameters (spatial frequency, profile shape, azimuth and so forth) and differ from the diffraction elements of the adjacent other bit line. A respective surface portion from the one bit line forms with the adjacent surface portion from the other bit line a bit pair representing a single bit of the information. In the case of that information strip, after manufacture it is possible to write an item of information once, wherein the optical characteristic of the surface portion is irreversibly altered in the operation of writing in one of the two surface portions of the bit pair.
It is known from EP 0 883 085 A1 that each surface portion of a single-line information strip has a plurality of partial surfaces which are occupied alternately with one of two different optical diffraction elements from an amount of n diffraction elements. The diffraction elements of a sub-amount involve the same grating parameters (spatial frequency, profile shape, azimuth etc) and differ from the diffraction elements of the other sub-amounts. Each surface portion differs from its two neighbors by the selection of the diffraction elements for the partial surfaces. The information of those information strips is established by manufacture and cannot be altered individually for each information strip.
WO 00/62237 A2 discloses an information strip with raster elements which are arranged in two lines and which are of equal size, wherein the one diffraction structure of the one line differs from the other diffraction structure of the other line and the grating grooves of the fine relief structures of the diffraction structures are oriented substantially parallel to the lines. A respective raster element from the one line represents a single bit of the information, with the adjacent raster element from the other line. With that information strip, after manufacture, an item of information can be written once, in which case the optical characteristic of the raster element is irreversibly altered in the operation of writing in one of the two raster elements of a bit.
A reading device must simultaneously read out the diffraction structures in both lines of the information strip, in which respect the reading field is to be oriented as exactly as possible with respect to the separation line between the two diffraction structures. The tolerance which can be accepted is not more than ±0.5 mm. Inlays in plastic cards or the like however have a typical tolerance of ±1 mm in relation to the edges.